Human bronchus, colon, duodenum, esophagus, and pancreatic duct cultured in a chemically defined medium provide an excellent in vitro system to study the metabolism of chemical carcinogens, including those found in tobacco smoke and the environment. Several classes of chemical carcinogens, polynuclear aromatic hydrocarbons, N-nitrosamines, hydrazines, aromatic amines, and mycotoxins can be metabolically activated by human tissues. Epithelial cell cultures initiated from human bronchus, bladder, and esophagus metabolized benzo(a)-pyrene (BP), a flatoxin B1 (AFB), and N-nitrosodimethylamine (DMN) into species that reacted with cellular DNA. The metabolic pathways leading to the formation of DNA adducts in explants and epithelial cell cultures have been defined for BP, 7,12-dimethylbenz(a)anthracene, AFB, and DMN. The adducts between these carcinogens and DNA in human tissues are essentially the same as those found in experimental animals in which the chemicals are carcinogenic. Interindividual differences in carcinogen-DNA binding values vary 50- to 150-fold.